1. Field of the Invention
The present invention relates generally to the field of optical disc memory systems. More specifically, the present invention relates to a two axis electromagnetic actuator, used for focusing and tracking in optical disc players/recorders.
2. Description of the Related Art
Optical data storage systems that utilize a focused laser beam to record and instantaneously playback information are very attractive in the computer mass storage industry. Such optical data storage systems offer very high data rates with very high storage density and rapid random access to the data stored on the disc. Optical recording and playback in programmable optical disc systems utilize laser light for both recording data on the disc and reading data from the disc. Recording and playback may be accomplished with the same laser source having different light power focused on the surface of the disc for the reading function and recording function, respectively.
In general, servo systems preferably utilize as input a bipolar error signal which carries the sign of the corrective action to be taken. Many methods have been proposed for the generation of radial i.e., tracking, and focus error signals. These error signals are often derived by optical means. A common optical method of generating focus error signals is based on the detection and analysis of a beam of laser light, which is reflected from the data storage plane of the disc.
Most present actuator driving combinations comprise a stationary part and a moving part. These systems generally comprise a coil, a permanent magnet and, in some instances, an iron yoke. These combinations may further be divided into two major groups: first, those having a moving coil and a stationary permanent magnet; and second, those having a moving permanent magnet and a stationary coil.
The moving coil configuration has a number of advantages as well as drawbacks. Besides the problems associated with breakage of the connector leads, which deliver electrical signals and power to the moving coil, the moving component may not make good thermal contact with the environment, owing to the movable suspension. The heat generated in the moving coil thus results in a temperature increase of the moving components which is undesirable if the moving components are adversely affected by elevated temperatures. Further, unless the coils are tightly wound, the dynamic resonance behavior of most moving coil designs tend to cause the moving coils to unwind. However, an advantage of the moving coil system is that the stationary permanent magnet may be made larger, thereby generating and applying a stronger magnetic field.
The opposite argument holds true for a moving permanent magnet combined with a stationary coil. Here, the coils may be made larger than in the moving coil configuration, and can be more tightly wound. This is advantageous in that a larger coil has a larger surface area for cooling. Thus, the heat removal from the coil poses a smaller problem, resulting in a higher permissible current through the coil. However, the maximum attainable magnetic field is decreased, because the maximum permitted size of the permanent magnet is smaller.
In order to attain a precise reading of the information stored on an optical disc, it is necessary to be able to move an objective lens in both a focusing (i.e., up-and-down) direction and a tracking (i.e., side-to-side) direction. Focus and tracking corrections may be affected by moving the objective lens in either the direction of the optical axis for focusing, or in a direction perpendicular to the optical axis for tracking. In some cases, separate magnetic fields are used for focusing and tracking, thus requiring the use of separate magnets, separate coils and separate yokes for each direction. However, the provision of a tracking assembly, separate from the focus assembly increases the size and difficulty of installation of the optical reading apparatus.
Actuators which utilize yokes can be very efficient in terms of acceleration for a given power dissipation. However, because fast access to the data stored on an optical disc requires high acceleration, the total dynamic mass of the actuator should be as low as feasible. The use of yokes made of iron or steel increases the total mass of the optical head, thus making the system response time slower.
Ito, et al., in U.S. Pat. No. 4,646,283 describe a system in which a focus coil is wrapped around an objective lensholder. Two pair of D-shaped tracking coils are bonded to the focus coil on laterally opposed sides of the lensholder. A pair of permanent magnets are secured to respective yokes on base of the actuator, in a manner to provide a magnetic gap between each permanent magnet and its associated yoke. In operation, when electric currents are applied to the coils, the coils produce fields which interact with the magnetic fields of the pair of permanent magnets. This interaction moves the objective lensholder in either the focus or tracking direction, depending upon the applied currents. The levels and polarities of the electric currents applied to the coils are controlled so that the holder is electromagnetically driven by the electromagnetic forces exerted across the magnetic gaps formed between each permanent magnet and its associated yoke. In this way, the motion of the actuator is controlled so that information recorded on the disc may be optically read. However, because the focus coils are wrapped around the lensholder and cover four of its six sides, only one path exists through which light may pass. This is undesirable in that it dictates that the laser light must enter the optical head from the bottom. Such a configuration necessarily increases the total height requirement of the actuator.